I'm really sorry to hear about this.
I help to run a support group for parents whose children identify as trans or non-binary. Based on our experiences and conversations we've published some basic advice for parents who've recently discovered their child's belief about their new identity, such as you've done by learning about it second-hand or sometimes when the child makes an announcement. In case it could be useful we've posted this here:
www.bayswatersupport.org.uk/toptentips
Early on, the key thing is to listen to your child without judgment, and without trying to bring the situation to a conclusion - by which I mean, it's very natural to want to respond with a conclusive 'yes' or 'no' but in this case, the goal is to keep options open, to encourage your child to think critically about sex and gender roles for a long time. That means that affirming your child straight away (ie by encouraging them to adopt girl's clothing etc) wouldn't necessarily be a great move because over time it can foreclose the process of reflection about themselves.
It's likely that he has read about trans identities online and often it can seem like a solution to other problems or discomfort the child is experiencing, about their developing body, their sexuality, or from a sense of not 'fitting in' (eg if they have an ASD diagnosis).
One reason why it is important to listen to your child first off, is that often the child will have read that their parents will reject them, that they won't understand, won't accept the child for 'who they really are.' When parents respond either angrily (that doesn't sound like you) or by rejecting the child's proposed identity out of hand, it plays into the narrative. Unfortunately, the transition narrative can have the effect of undermining family bonds. Given that this is happening in early adolescence, whose purpose is to enable the child to separate from their parents, it is even in usual circumstances a vulnerable time for the child-parent relationship. But if not handled carefully by parents, the child can separate before they are really ready - there are many voices online that will offer them affirmation, encouragement in what they are doing.
If your boy has come out at school, I would suggest having a meeting with them to find out what they know about it. If they have nurtured this idea without letting you know, then that is plainly against the child's best interests.
Difficult to say much without knowing more. Usually this is something that will occupy your child for a long time - a year or more. Very best of luck. Do by all means get in touch via our site or forum if that could be helpful.